Doka�te, �e plat�
$$
\sum _{n=1}^{\infty }{\frac {\left (-1\right )^{n}
\left (2\,{n}^{2}+1\right ){x}^{2\,n}}{\left (2\,n
\right )!}}=\cos(x)-{\frac {\sin(x)x}{2}}-{\frac {{
x}^{2}\cos(x)}{2}}, \quad x \in R.
$$